Abstract: A commercial CoMo/Al2O3 hydrodesulfurization (HDS) catalyst was sulfurized in a fixed-bed micro-reactor under various pressures. The sulfurized catalyst samples were characterized by high-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), as well as HDS activity evaluation. The TEM results show that the number of stacking layers and the length of MoS2 nano-particles increase with the sulfidation pressure, which is helpful to weaken the interaction of metal-support. The XPS results show that the sulfur content and the sulfidation degree of the sulfurized catalyst increase with the sulfidation pressure, which benefits the improvement of catalyst activity. Moreover, supplemental sulfidation may happen during the HDS reaction due to reaction pressure is higher than sulfidation pressure sometimes, and it is shown from the XPS results of catalysts reacted under 4.0 MPa for 18 h. The evaluation data of catalyst performance agree well with the characterization results, under a reaction temperature of 360 ℃ and a reaction pressure of 4.0 MPa, catalysts exhibit high activities, the maximum HDS rate reaches 99.5%.